1. Field of the Invention
The present invention relates to a high frequency power amplifier used in microwave and millimeter wave band communication devices for mobile communications, satellite communications and the like.
2. Background Art
In microwave communication systems using multicarrier signals or modulated wave signals, such as recently developed CDMA system, the effect of distortion generated by the non-linearity of a high frequency power amplifier must be minimized. On the other hand, it has been known that if a resonant circuit, which becomes an open load or high-impedance load to the secondary harmonic, is located at the input side, the efficiency of a high frequency power amplifier can be improved (for example, refer to Japanese Patent Application Laid-Open No. 2002-164753). In a high-output power amplifier for mobile communications and satellite communications, to obtain large power output, a multi-finger transistor, with a plurality of transistor cells electrically connected in parallel to enlarge gate width, is employed.
FIGS. 19 and 20 are circuit diagrams showing reference examples of high frequency power amplifiers having resonant circuits located at input sides. Transistor cells 11 are electrically connected in parallel, an input-side matching circuit 13 is connected to the gate electrodes of the transistor cells via adjusting lines 16 and bonding wires 12, and an output-side matching circuit 15 is connected to the drain electrodes of the transistor cells via bonding wires 14. In the circuits shown in FIGS. 19 and 20, resonant circuits 17 are located before the input-side matching circuit 13. The resonant circuit 17 shown in FIG. 19 is an open load to the second harmonic; and the resonant circuits 17 shown in FIG. 20 are high-impedance loads to the second harmonic.
In the circuit shown in FIG. 19, since the resonant circuit 17 is connected after gate electrodes have been collectively connected to the input-side matching circuit 13, phase differences occur between the cells when the input-side matching circuit 13 is viewed from the gate electrode of each transistor cell 11. Therefore, the second harmonic load, viewed from the gate electrode of each transistor cell 11, cannot be an identical high-impedance load for each transistor cell. When a multi-finger transistor is used, the input impedance of the transistor is inevitably significantly lowered. For this reason, in the circuit shown in FIG. 20, since propagation loss attenuates reflected waves from the resonant circuit 17, the second harmonic load cannot be controlled as an open load. Therefore, the circuits shown in FIGS. 19 and 20 have a problem of poor distortion characteristics.